Endovascular infections include infective endocarditis, mycotic aneurysm, vascular catheter and hemodialysis site sepsis, and vascular graft and prosthesis infections. The incidence of endovascular infections has increased dramatically over the past decade and now represent a significant proportion of all serious infections. In light of the increasing use of indwelling biomaterials such as artificial cardiac valves, prolonged use of indwelling catheters, and endovascular stents, the incidence of endovascular infections will likely continue to grow. Moreover, multiple antibiotic-resistant pathogens which often cause these infections (eg., Staphylococcus aureus) are now being isolated with disturbing frequency. Collectively, these facts underscore the urgent need for greater understanding of host defense against endovascular infection. A clearer concept of host defense in this regard may reveal opportunities for its augmentation, or novel targets for prevention or treatment of endovascular infection. Platelets are among the earliest and most predominant cells at sites of endovascular infection. Dr. Yeaman and his colleagues have recently isolated a family platelet microbicidal proteins (PMPs), which may contribute to the antimicrobial properties of platelets. However, their release from platelets, comparative structures and microbicidal activities have not been established. Therefore, the Specific Aims of this proposal are designed to address the following questions: i) do microorganisms or platelet agonists associated with endovascular infection elicit PMP release?; ii) what is the structural relationship between secreted and non-secreted PMP forms?; and iii) what are the comparative microbicidal capacities of differing forms of PMP? They will determine where PMPs exist in the platelet using granule isolation and immunolocalization methods. They will use a panel of microorganisms and platelet agonists relevant to infection to examine which elicit PMP release. They will determine and compare the compositions, primary structures, and conformations of representative secreted and non-secreted PMP forms using protein biochemistry and molecular biological techniques. They will also determine and compare the microbicidal spectra, potencies, kinetics, and interactions of secreted and non-secreted PMP forms in vitro using a panel of common bloodstream pathogens along with PMP-sensitive and -resistant S. aureus pairs as probes. The long range goal of these investigations is to advance our understanding of the role of PMPs and platelets in host defense against endovascular infection. This knowledge may provide novel strategies for preventing or managing endovascular infections. Furthermore, these studies may provide information applicable to broader issues of pathogenesis, such as microbe interaction with biomaterials. In addition, these studies may uncover important new information regarding PMP structure and function that may serve in designing potent new antimicrobial agents. These potential applications underlie the focus of this project.